Green strength of binder-free ceramics

The Discrete Element Method (DEM) is used to investigate the compaction of ceramic powders by focusing on the role of primary particle and aggregate sizes. Hard aggregates of primary particles are represented as a collection of spherical particles bonded together by solid necks, which may break during compaction. Numerical simulations are performed to investigate the effect aggregate microstructure both on compaction and on the green strength of the ceramic compact. Tensile green strength originates only from van der Waals adhesion, without any action of a binder. It is shown that in such conditions, the green strength is inversely proportional to the size of the primary particles. The size of aggregates also plays a role, with smaller aggregates leading to larger green strength. The results of the simulations are compared to experimental data obtained on dioxide uranium aggregated powders, confirming the particle size effect.     

Discrete Element Simulations of the Compaction of Aggregated Ceramic Powders

The compaction behavior of aggregated ceramic powders is investigated using the discrete element method (DEM). We use specific contact forces that represent bonds between primary particles to model aggregates. The bonds have the possibility to break when their strength is exceeded, hence reproducing the crushability of aggregates. The strength of individual aggregates has been investigated using compression between platens. A number of aggregates are then introduced in a periodic simulation box to model close-die compaction. It is shown that the densification of the powder is mostly due to aggregate attrition and crushability, coupled with rearrangement. The effect of bond size, aggregate strength, and morphology on the powder macroscopic behavior is assessed.     

Dependence of Compaction Efficiency in Dry Pressing on the Particle Size Distribution

The compact densification with pressing pressure (compaction efficiency) was determined to be sensitive to the particle size distribution. For the three types of alumina powders used in this research, the compaction efficiency increased with increasing particle size. It has been demonstrated that if the compact density versus log (pressure) has a linear relationship for any two types of powders, so do the blends of the two powders. A model is proposed which can predict the compaction efficiency of a binary particle system based on the Furnas particle packing model and consider the packing efficiency as a function of forming pressure. The composition of the binary mixture at which the highest density is obtained under high pressures is also the composition having the largest compaction efficiency. When coarse particles were added to this composition, the compaction efficiency slowly decreased, and when fine particles were added, the compaction efficiency rapidly decreased. For a continuous particle size distribution, the highest compaction efficiency is related to the average value of -log (porefraction).     

粗集料对水泥稳定石屑击实和强度特性的影响研究

为提高水泥稳定石屑的强度且便于工程操作,提出在水泥稳定石屑混合料掺入单一粒径的粗集料。试验研究粗集料掺量对混合料击实特性和强度特性的影响。分别将粒径规格为4.75mm^9.5mm和9.5mm^19mm的粗集料掺入到水泥稳定石屑中,设定不同粗集料和石屑比例,测试混合料的击实曲线、7d无侧限抗压强度和间接抗拉强度,并与不掺入粗集料的水泥稳定石屑相比较。试验结果发现:当粗集料掺量不超过50%时,两类掺入粗集料的水泥稳定石屑级配仍属于悬浮密实型,混合料压实曲线突起部分随粗集料掺量增加逐渐变窄,表明混合料对含水率逐渐敏感。掺入粗集料后混合料强度和抗缩裂性能均有所提高,粗集料粒径规格越大,掺入量越大,性能提高越明显。     

Porosity, pore size distribution and in situ strength of concrete

In this study, in situ strength of concrete was determined through compression test of cores drilled out from laboratory cast beams. The apparent porosity and pore size distribution of the same concrete were determined through mercury intrusion porosimetry, performed on small-drilled cores. The normal-strength concrete mixes used in the experimental investigation were designed to exhibit a wide variation in their strengths. To ensure further variation in porosity, pore size distribution and strength, two modes of compaction, two varieties of coarse aggregates, different levels of age, curing period and exposure condition of concrete were also introduced in experimental scheme. With the data so generated, an appraisal of the most frequently referred relationships involving strength, porosity and pore size of cement-based materials was carried out. Finally, a new empirical model relating the in situ strength of concrete with porosity, pore size characteristics, cement content, aggregate type, exposure conditions, etc., is presented.     

Effect of compaction pressure on the sinterability of submicron powders of tetragonal zirconium dioxide

Conclusions We studied the effect of compaction pressure on the pore structure of the paniculate compacts obtained using two types of agglomerated submicron powders of tetragonal zirconium dioxide, on the structure evolution during the sintering process, and on the strength of the obtained material. It was established that the characteristics of the agglomerates present in the powders have a significant effect on their behavior during compaction and sintering. At a given compaction pressure, the powders having weaker agglomerates densify up to a higher density and give a more uniform distribution of pores in the preform. The low-density compacts obtained using agglomerated powders having a high specific surface area sinter faster and attain high strength levels at a lower temperature; however, the sintered materials obtained from such compacts contain several structural defects in the form of large pores and have a lower strength. The uniformity of the distribution of pore volume with respect to size (or the specific content of the interagglomerate pores) forms the main criterion of the quality of particle packing in the compacts obtained from agglomerated powders. The compacts having a low content of the interagglomerate pores give a defect-free dense and strong material after sintering. The presence of the anion impurities in the original powders leads to a decrease of density during the sintering process after the attainment of a threshold density at which formation of closed porosity occurs. Pressure sintering (HIP) forms an effective method of suppressing the decrease of density.Translated from Ogneupory, No. 2, pp. 5–11, February, 1993.     

Particle Size Distribution and Its Relation to Sintering—A Case Study for UO2 Powders

It is generally recognized that the particle size distribution (PSD) of powders plays a very significant role in powder compaction and sintering. However, particle size distribution data are generally presented in the form of one number (mean diameter). In the case of PSD data obtained by sedimentation, analysis of the whole curve reveals some interesting information. The type of information obtained by such an analysis and its application to an analysis of compaction and sintering is discussed using the case of uranium dioxide powders as an example.     

Simulations of vibrated fine powders

During the past few decades, several studies have been conducted to understand the behaviour of powders in vibrated beds. This paper introduces a technique of incorporating the agglomeration and deagglomeration phenomena in the simulation of vibrated fine powders. Two-dimensional direct simulations are performed using 300 spheres 2.99 mm in diameter in a trapezoidal container vibrated vertically at an amplitude of 2.5 mm and 20 Hz frequency as preliminary conditions. Under non-cohesive conditions, the results are in agreement with those found in the literature. As a preliminary effort to predict the behaviour of cohesive fine powders under vibrated conditions, agglomeration and deagglomeration processes are modelled as the formation and destruction, respectively, of interparticle bonds during particle collisions. Two parameters used to model agglomeration and deagglomeration are the ease of cohesion and cohesivity of the powder. Dependencies of these parameters on certain physical properties of cohesive powders have been suggested. Simulation results reveal two aggregate populations, one with uniform size aggregates and another population with multi-sized aggregates. The former aggregates were more prevalent in weakly cohesive powders while the latter in highly cohesive powders. Interesting macroscopic bed behaviour such as alternating cycles of agglomeration and deagglomeration were also observed. Further work is needed in which the aerodynamic forces are taken into account and cohesion mechanisms at the particle surface are modelled.     

Influence of Aggregates on Sintering

Alumina powders containing controlled amounts of hard aggregates exhibit isothermal shrinkage rates which decrease markedly with increasing aggregate content. Sintered densities achieved after 1- and 24-h anneals decreased linearly with aggregate content. The microstructures of regions near aggregates and within aggregates showed that the compacts sinter differently inside and outside the aggregates.     

混凝土中邻近集料表面最近间距分布的计算机模拟

混凝土中邻近集料问界面过渡区的相互影响程度、混凝土中原生裂纹的尺度范围以及中心质假说中集料效应圈的范围等都涉及到混凝土中最邻近集料表面间距分布的问题。由于常规的实验方法无法给出混凝土中集料空间分布的信息,同时以往的计算机模拟方法由于采用随机分布方式分布粒子,导致无法得到较高集料体积分数的模型混凝土结构,而采用具有粒子动态混合密实功能的SPACE系统,模拟了高集料体积分数混凝土的结构。在假定模型混凝土中集料的最小粒径为1mm的前提下,以符合Fuller分布为例,研究了模型混凝土中集料粒径分布和集料体积分数对邻近集料表面最近间距分布的影响。     

Material characteristics affecting formcoke

The influence of aggregate and binder phase characteristics on formcoke products has been studied by investigating the compaction kinetics of the system and determining the mechanical strength of the briquettes produced. The char phase was characterized in terms of density, hardness and porosity and the binder phase with respect to rheological properties. Results indicate that binder phase fluidity affects compaction viscosity during the particle flow stage of compaction and that char porosity influences final briquette bulk density by affecting the amount of total compaction required to obtain a given bulk density. In general, increased total compaction was shown to result in higher product bulk density with the attendent higher gross composite strength. The latter relationship was seen to be approximately linear over the range of bulk porosity found in this study. A higher briquette strength was found for systems with aggregates carbonized at lower temperatures. Similarly the binder phase fluidity was also seen to affect briquette strength, higher fluidity resulting in higher strength. It was concluded that this was due to increased binder penetration in the highly porous char carbonized at lower temperatures. With no significant pore structure in the aggregate, as found with high temperature char, briquette strength was seen to become approximately constant for the three binder coals used.     

长春花植物细胞团的粒径分布及其模糊数学模型

对长春花植物细胞团前10代粒径分布进行了研究,其平均粒径和分布的分散程度经历由大到小,再由小到大,最后达较稳定值的过程.针对模糊群子模型进行参数估计,拟合结果表明,植物细胞团粒径分布遵从能反映分散与成团矛盾作用的模糊群子模型.     

Characterization and cold compaction of polyether-etherketone powders

This study deals with the characterization and cold compaction of polyetheretherketone (PEEK) powders. Four different types of PEEK powders which are commercially available from Imperial Chemical Industries (ICI) were characterized for density, crystallinity, particle size, particle size distribution, and particle morphology. Fine and coarse size powders of a low viscosity grade (150PF and 150P) and similar powders of a high viscosity grade (450PF and 450P) were processed. Compaction was successful at room temperature using the 150 grade powders but not with the 450 grade powders. Compressibility curves were obtained at room temperature for the 150 grade powders and their post-compaction viscoelastic recoveries were measured. The coarse and fine size 150 grade powders reached relative plateau densities of 95.8% and 95.1%, respectively, when compacted at pressures exceeding 300 MPa. These high densification values at room temperature were associated with minimal post-compaction viscoelastic recoveries and as-compacted strengths as high as 8.9 MPa. A modified densification parameter (DP*) was developed based on the plastic deformation of the crystalline regions alone. This new dimensionless parameter, DP*, demonstrated a better fit of all the compaction data. The transverse rupture strength and the green density data are presented and explained in terms of this DP*.     

再生骨料对混凝土力学性能及耐久性影响的研究

针对再生骨料由于本身结构缺陷的原因,造成混凝土力学性能和耐久性不良的问题,采用再生骨料裹浆预处理和密实骨架堆积配合比设计方法,弥补再生骨料的结构缺陷,提高混凝土密实程度,使得再生骨料混凝土的抗压强度和抗碳化性能、抗氯离子渗透性能得到有效改善。     

Effect of agglomerate size on extrudability of titania pastes

Pastes prepared from three high surface area titania powders were tested via torque and capillary rheometry to explain the different flow behaviour during mixing and extrusion processing. The Benbow analysis was used to calculate the main extrusion parameters, including bulk yield and shear stresses, and to compare the processability of the extrudable pastes. A critical water fraction was found for all powders and related to extrudability. The results show that the flow properties of the pastes are strongly influenced by the particle size distribution and the shear stability of large aggregates of different dimensions in the paste microstructure.     

高含量β-胡萝卜素微胶囊干粉流动性的考察及其影响因素

The flowability of five kinds of microencapsulation powders,with differentβ-carotene contents and by two alternative particle-forming technologies i.e.spray-drying and starch-catching beadlet technology,was meas- ured.The actual flow properties of the five powders were compared based on bin-flow test,and three flow indexes (Hausner ratio,repose angle and flow index)were measured.It was found that the repose angle is the most suitable index to reflect the flowability of these powders for the particle properties would not be altered due to compaction or tapping during the measuring process.Particle size and particle size distribution play most important roles in the flowability of these granular materials,which was also influenced by other factors like shape,surface texture,sur- face roughness,etc.Microcapsules with wall material of gelatin and a layer of modified starch absorbed on the sur- face showed excellent flowabilities and good mechanical properties,and they are favorable for tabletting to supply β-carotene.     

Characterization of Textured Ceramics by Electron Paramagnetic Resonance Spectroscopy: II, Formation and Properties of Textured MgO

A method for the formation of macroscopic quantities of faceted, nanophase MgO precursor powders has been developed that permits a degree of control over the particulate size distribution. By employing powders of this type it was possible to form textured MgO ceramics by means of cold-pressing and sintering. The theoretical basis for the application of EPR techniques to texture analysis as developed in Part I of this work was used to establish the nature of the preferred orientation in these MgO ceramics. Both EPR and X-ray analyses showed that preferred orientations of the 〈111〉 axes of the MgO microstructural grains occur along or near the direction of force used to compact the green body. EPR results also show that this texture develops during the sintering process and that no preferred orientation effects are present in the unsintered body formed by cold-pressing the faceted, nanophase MgO powder. Effects of parametric variations such as sintering temperature, compaction force, and faceted particle size on the degree of preferred orientation in these sintered MgO ceramics were investigated using EPR techniques. The results show that the amount of texturing increases with increasing compaction force or temperature up to a limiting value and that it also increases with decreasing faceted particle size.     

Measurements of pore size distributions in cements,aggregates and soils

Pore size distributions, measured by mercury intrusion, have been used to better explain, and even predict, the behavior of a variety of construction materials, viz. hydrated portland cement, portland cement concrete aggregates, compacted fine-grained soils, and shales. In the cases of the cement and aggregates (including the shales), the response of interest was durability under the simulated environmental changes of an inservice position. For the soils, the compaction characteristics and their water permeability after compaction and saturation were the items studied. In all cases, examination of the pore size distribution aided substantially in understanding the physical condition (or fabric) of the material. In addition, several examples of statistically valid prediction equations using pore size parameters as independent variables are cited.     

The powder properties of poly(vinyl chloride) resins

The properties of powders depend mainly upon the characteristics of the individual particles and their interactions. In the case of PVC resins, particle characteristics vary broadly depending upon the manufacturing process used or variables within a given process. In this paper, the effects of grain size, size distribution, grain shape, degree of compaction and static on the bulk density and flow properties of PVC resins are studied. Results indicate that grain shape mainly affects bulk density while grain size and distribution mainly affect powder flow. The degree of compaction and the amount of static on the resin grains are variables which can overwhelm other powder properties.